Method for making ceramic fibers from a water soluble pre-ceramic polymer solution

ABSTRACT

A method of making ceramic fibers from a water soluble pre-ceramic polymer solution by forming fibers from a water soluble pre-ceramic polymer solution, drying the formed fibers at a first temperature ranging between 600° C.-750° C. at a temperature heating rate of about 1° C. per minute. Next, raising the first temperature to a second temperature ranging from 800° C.-1000° C. at about 5° C. per minute, and increasing the second temperature to still a third temperature of about 1200° C. at about 10° C. per minute and holding at the third temperature for about 1 hour. Finally, the fibers are cooled to room temperature at a rate of about 5° C. to 30° C. per minute.

This is a continuation of application Ser. No. 08/307,218 filed Sep. 16,1994 now abandoned which is a continuation of application Ser. No.08/061,134 filed May 13, 1993 now U.S. Pat. No. 5,437,852.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to a process of synthesizinginorganic polymers, and more particularly to the production ofwater-soluble aluminum and yttrium based polymers for use in thin films,fibers, and composite matrices.

2. Description of the Related Art

Ceramics can be prepared from a number of precursor states such asmolten liquid, powder, vapor, and polymer. The polymer based process,despite its relatively short history, has been the subject of muchinterest because of its potential economic benefits. Compared to vacuumdeposition techniques, it is a far more cost-effective way of depositingceramic coatings. Antireflective coatings on window glass and siliconsolar cells, planarization layers on ultrasmooth surface mirrors, andsuperconducting thin films on microwave cavities are some examples. Inaddition to the economic benefits, the technique allows fabrication ofshapes that are impossible by other methods. For example, theviscoelasticity of high molecular weight polysiloxane and zirconiumacetoacetonate derivatives allows the fabrication of high modulus SiO₂and ZrO₂ fibers, respectively.

Despite the success, the polymer route has not yet gained industry-wideacceptance mainly because of the following two problems: 1) difficultyin handling, or intractibility, and 2) low ceramic yield. Theintractibility arises due to the improper choice of precursors. The mostoften used precursors for polymer synthesis are metal alkoxides orderivatives of metal alkoxides, all of which are hygroscopic. Use oflarge quantities of anhydrous organic solvents and handling in inertatmospheres are therefore unavoidable. Low ceramic (or char) yield isanother problem. Most pre-oxide polymers have ceramic yields far below30% and no polymer with a yield greater than 40% has been reported. Thelow yields cause large drying and sintering shrinkages which invariablylead to poor dimensional control.

Therefore, it is desirable to have a process for synthesizingwater-soluble pre-ceramic polymers of high char yields from inexpensiveprecursors. Such a process would be of tremendous economic value.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems with the priorart as well as others by providing a method for making a water Solublepre-ceramic polymer. The water soluble pre-ceramic polymers of thepresent invention are suitable for impregnating preform tubes. Furtherimpregnations are attainable beyond 18% by a unique approach withaddition of alcohol.

Alternately, fibers are fabricated directly from the pre-ceramic polymerby adjustment of the polymer solution viscosity to exceed 10" centipoise(cps).

Advantageously, several compounds including nitrate stabilized zirconiasol, acetate stabilized zirconia sol, Y₂ O₃ stabilized ZrO₂ powder, andY₄ Al₅ O₁₂ may be added to the pre-ceramic polymer for particularapplications. Also, an item of hot pressed Y₂ O₃ stabilized ZrO₂ may becoated with a highly concentrated polymer solution of the presentinvention.

The method of the present invention makes a water soluble pre-ceramicpolymer by heating a hydrated metal salt in a furnace to a temperatureabove the melting point of the hydrated metal salt. A flow of air isprovided above the melt to drive the polymerization reaction forwarduntil a predetermined weight loss of the pre-ceramic polymer isobtained.

An object of the present invention is to synthesize water-solublepolymers in which major portions consist of metal and oxygen atoms.

Another object of the present invention is to synthesize water-solublepre-ceramic polymers from inexpensive precursors.

Yet another object of the present invention is to impregnate fiberpreforms with polymer solutions of the present invention as precursorsto matrices.

Still another object of the present invention is to spin ceramic fibersof fine diameter and microcrystalline grains from the polymers of thepresent invention.

Still a further object of the present invention is to deposit thin filmson ceramic substrates from the polymers of the present invention.

The various features of novelty characterized in the present inventionare pointed out with particularity in the claims annexed to and forminga part of this disclosure. For a better understanding of the invention,the operating advantages attained by its uses, reference is made to theaccompanying drawings and descriptive matter in which a preferredembodiment of the present invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a photograph of an Al₂ O₃ polymer powder prepared inaccordance with the present invention and on its right a solutionprepared by dissolving the polymer in water;

FIG. 2 is a pair of x-ray diffraction patterns of a pre-Al₂ O₃ polymerplotted as a function of 2e, which is the angle between the transmittedand diffracted x-rays. It indicates that the as-prepared polymer isamorphous and that after heating at 1200° C. for 1 hour crystallizes toα-Al₂ O₃ ;

FIG. 3(a) is a combined graph of a differential thermal analysis (DTA)and a thermogravimetric analysis (TGA). It indicates that the polymercrystallizes from an amorphous state to e-Al₂ O₃ at 776° C. and α-Al₂ O₃at 1082° C., respectively;

FIG. 3(b) is another combined graph of a DTA and TGA. It shows that thepolymer has a ceramic yield as high as 80% on a weight percent basis;

FIG. 4 is a plot of the viscosity in centipoise (cps) of a polymersolution as a function of polymer concentration in water;

FIG. 5 is a plot of the weight gain of a ceramic matrix composite tubeas a function of the number of impregnations with a pre-Al₂ O₃ polymersolution;

FIG. 6 is a Scanning Electron Microscope microphotograph of across-section of an Al₂ O₃ fiber/Al₂ O₃ matrix composite;

FIG. 7 is a photograph of an Al₂ O₃ fiber being pulled from aconcentrated pre-Al₂ O₃ polymer solution;

FIG. 8(a) is a photograph of as-drawn Al₂ O₃ fibers from a polymersolution;

FIG. 8(b) is a photograph of the same fiber after heat treatment at1200° C.;

FIG. 9(a) is an SEM micrograph of the same fiber shown in FIG. 8(b); and

FIG. 9(b) is an SEM micrograph of a fractured surface.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention resides in the ability of hydrated metal salts toundergo polycondensation reactions at temperatures above their meltingpoints. The method comprises the steps of heating metal salts totemperatures above their melting points and holding the melts in a wellventilated oven to drive off hydrated water and the ligands. Theconstant flow of fresh air above the melt in the range of 200 to 2000cubic centimeters per minute and preferably between 500 and 1000 cc/mindrives away the ligands as they are expelled from the growing polymerchains. This drives the polymerization reaction forward and increasesthe lengths of the polymers. The heating is continued until all the freeligands are driven off and only solid polymers are left. This isdetermined by the amount of weight lost during heating. The overallprocess may be represented by the following chemical equation: ##STR1##where M is a metal such as aluminum or yttrium, and n is the number ofrepeating units in the polymer chain. The equation is not balanced forsimplicity. Care must be exercised so as not to overheat the polymers.The temperature above the melting point of the polymers should notexceed 100° C. Some ligands must be left attached to the polymers asside groups for them to be water-soluble. A predetermined weight loss ofless than 73 weight percent for Al₂ O₃ polymers and 36 weight percentfor Y₂ O₃ polymers provides for this. The solid polymers readilydissolve in heated water at a temperature higher than about 60° C. togive highly concentrated solutions. The polymer solutions have shelflives in excess of 6 months at room temperature and at a pH below 4.

The following examples are illustrative of test results that demonstratethe present invention:

EXAMPLE 1

Al₂ O₃ Precursor Polymer

1488 g of Al(NO₃)₃ 9H₂ O powder is spread evenly in a rectangularceramic tray whose dimensions are approximately 12"×10.5"×2.5". The trayis placed in a large drying oven equipped with a convection fan. Thetemperature of the oven is raised to a temperature between -110° and190° C. and kept at that level to allow the melting of the salt,expulsion of hydrated water and nitrate ligands, and polymerization totake place. Heating is continued for 48 hours or until the total weightloss is between 71.5% and 73% on a weight percent basis. The formedwhite, amorphous polymer readily dissolves in water at temperatureexceeding 60° C. (FIG. 1), and has a ceramic yield of 80% by weight,note FIG. 3(b), and crystallizes to fully crystalline α-Al₂ O₃ at 1080°C. as evidenced by FIGS. 2 and 3(a).

EXAMPLE 2

Y₂ O₃ Precursor Polymer

1.0 g of Y(NO₃)₃ 6H₂ O is placed in a 6 inch long Al₂ O₃ boat. The boatis then placed inside a well ventilated oven and the temperature israised to 275° C. The temperature is maintained until the total weightloss is between 34% and 36% on a weight percent basis. The formed white,solid polymer has a ceramic yield of 40% by weight.

EXAMPLE 3

Al₂ O₃ Matrix Composite Fabrication

The same procedure as described in Example 1 is followed for thepreparation of a pre-Al₂ O₃ polymer solution. The solution concentrationis adjusted so that the total polymer to water weight ratio is 1:2. Theviscosity of the solution is approximately 50 centipoise, refer to FIG.4, and the density equals 1.47 g/cc. This polymer solution is used tovacuum impregnate an Al₂ O₃ preform tube whose dimensions are 1.5" I.D.,1.75" O.D., and 1" in length. The tube is prepared by filament windingAlmax, a registered trademark of Mitsui Mining Co., Japan, Al₂ O₃fibers. The impregnated tube is dried in air at 110° C. for 24 hours andcalcined at 500° C. The process of impregnation and calcination isrepeated 15 times with 2 intermittent (after five repeats) 1100° C.sintering cycles. The porosity of the tube after 15 impregnation cyclesis 18%. It has been found that further impregnations do not decrease theporosity beyond 18%. It can be decreased further only by an alcoholaddition, as described in Example 4.

EXAMPLE 4

Al₂ O₃ Matrix Composite Fabrication--Alcohol Addition

The same fabrication procedure as in Example 3 is followed to prepare asolution whose concentration based on Al₂ O₃ content is 25% by weight.7.9 g of methanol (MeOH) is added to 160 g of this solution and stirred.The viscosity of this solution is 14.8 cps. A solution containing onlywater possesses a viscosity of 46.6 cps. The concentration of alcohol isonly 5%, but the drastically reduced viscosity and surface tension ofthe solution greatly improves the penetrability of the polymer inpreforms. With the modified solution, porosity levels as low as 13% canbe achieved, as shown in FIG. 5. This composite tube has a densemicrostructure, as seen in FIG. 6.

EXAMPLE 5

Al₂ O₃ --ZrO₂ Matrix Composite Fabrication

The same procedure as in Example 1 is followed to prepare a pre-Al₂ O₃polymer solution. The concentration is adjusted so that the polymer towater ratio is 1:1.75 by weight. 95 g of nitrate stabilized zirconia sol(Zr 10/20, a registered trademark of Nyacol, Inc., Ashland, Mass.) whoseconcentration is 20% by weight is added to 262.5 g of this solution. Theformed mixture is clear and colorless. For long term stability, thesolution pH must be below 2. This mixture is used to fabricate Al₂ O₃--ZrO₂ matrix composite tubes by following the same impregnation andheat treatment cycles described in Example 3.

EXAMPLE 6

Al₂ O₃ --ZrO₂ Matrix Composite Fabrication

The same preparatory procedure as in Example 5 is followed, except forthe replacement of ZrO(NO₃)xH₂ O for the nitrate stabilized sol.

EXAMPLE 7

Al₂ O₃ Fibers

The same materials and procedure as in Example 1 are followed to preparea pre-Al₂ O₃ polymer solution. The solution is then concentrated withmild heat (60° to 80° C.) until the room temperature viscosity exceeds106 centipoise. A glass rod or stainless steel spatula is inserted intothe polymer mass and quickly pulled up. Depending on the pulling speedand the viscosity of the solution, fibers between 10 and 100 μm indiameter can easily be pulled, see FIG. 7. Fiber lengths in excess of 2m can be achieved readily. FIG. 8(a) shows as-spun fibers that are fineand flexible. The as-spun fibers are dried and calcined by heating themin an electrical box furnace at 1° C./min to 600° C. From 600° C., thetemperature is raised to 800° C. at 5° C./min. Following an isothermalhold at 800° C. for 1 hr., the temperature is raised to 1200° C. at 10°C./min. It is held at 1200° C. for 1 hr and then cooled to roomtemperature at SaC/min. FIG. 8(b) shows the heat treated α-Al₂ O₃fibers. It clearly shows that the heat treatment did not damage fiberintegrity. FIG. 9(a) shows an SEM micrograph of a sintered α-Al₂ O₃fiber. The 10 μm fiber is fully dense and consists of grains that areapproximately 0.2 μm in diameter.

EXAMPLE 8

Al₂ O₃ --(Y₂ O₃)--ZrO₂ Fibers

The same preparatory procedure as in Example 7 is followed, except forthe addition of fine Y₂ O₃ stabilized ZrO₂ powder. The powder isprepared by dispersing commercially available 8 mole % Y₂ O₃ stabilizedZrO₂ (Zircon Products, Inc., New York) in water at a pH of 4. Thedispersion is covered with a polyethylene film and allowed to stand for1 week. Particles finer than 0.2 μm in diameter are collected byclassification and added to the alumina polymer solution. The fiberspinning and heat treatment procedures are exactly the same as thosedescribed in Example 7.

EXAMPLE 9

Al₂ O₃ --Y₃ Al₅ O₁₂ Fibers

The same preparatory procedure as in Example 8 is followed, except forthe replacement of ZrO₂ particles with Y₃ Al₅ O₁₂. Y₃ Al₅ O₁₂ powder isprepared by dissolving 86.1774 g of Y(NO₃)₃ 6H₂ O and 140.6739 g ofAl(NO₃)₃ 9H₂ O in 200 g of distilled water. In a separate beaker, 150 gof aqueous NH₃ is mixed with 300 g of distilled water. While vigorouslystirring, the first solution is poured into the ammonia solution. White,gelatinous particles precipitate immediately upon mixing. The powdersuspension is then washed 4 times with excess distilled water. It isthen calcined at 800° C.

EXAMPLE 10

Al₂ O₃ Thin Films

The same materials and preparatory procedure used in Example 1 arefollowed to produce a highly concentrated polymer solution. The polymerconcentration is between 50% and 60%. A polished piece of hot pressed Y₂O₃ stabilized ZrO₂ is then dipped into the solution and slowly pulledupward. The coated piece is then dried at room temperature for 24 hr andheated to 500° C. at 1° C./min. The temperature is then raised to 1200°C. at 5° C./min and held for 1 hr. The process of dipping, drying, andheating is repeated until a sufficiently thick coating is obtained.

The foregoing examples are intended for illustrative purposes and arenot meant to limit the present invention. For example, other fiberforming techniques such as gas jet fiberization or a spinnerette may beemployed for making fibers in Example 7. Similarly, other coatingtechniques may be used in Example 10.

The following Table 1 provides several precursors tested including theweight loss and water solubility.

                  TABLE 1                                                         ______________________________________                                        PRECURSOR    WEIGHT LOSS*                                                                              WATER SOLUBILITY                                     ______________________________________                                        AlCl.sub.3 6H.sub.2 O                                                                      18.3%       Yes                                                               24.4        Yes                                                               48.0        Yes                                                               59.0        Yes                                                               61.4        Yes                                                               61.66       Yes                                                               74.7        No                                                   Al(NO.sub.3).sub.3 9H.sub.2 O                                                              63.5        Yes                                                               64.3        Yes                                                               66.5        Yes                                                               67.3        Yes                                                               68.3        Yes                                                               71.1        Yes                                                               72.6        Yes                                                               72.9        Yes                                                               73.0        No                                                                73.6        No                                                                82.9        No                                                   3Y(NO.sub.3).sub.3 6H.sub.2 O                                                              14.2        Yes                                                               17.4        Yes                                                               24.5        Yes                                                               27.4        Yes                                                               35.1        Yes                                                               36.1        No                                                   Y(NO.sub.3).sub.3 6H.sub.2 O                                                               55.8        Yes                                                  5Al(NO.sub.3).sub.3 9H.sub.2 O                                                             57.6        Yes                                                               58.6        Yes                                                               59.8        No                                                                72.8        No                                                   ______________________________________                                         *Weight loss during heat treatment above 110° C.                  

The process of the present invention is applicable to all hydrated metalsalts that melt at low temperatures. For example yttria and yttriumaluminate garnet (YAG) polymers have been successfully processed by thepresent invention.

The pre-ceramic polymers are easily employed in any of the followingcommercial applications as well as others.

The low viscosity of the highly concentrated pre-ceramic polymersolutions is ideal for coating or filling micropores. Greater than 10volume percent alumina solutions with a viscosity less than 100centipoise have been prepared. This solution is used as a matrixprecursor.

The wettability of the pre-ceramic polymer on most commerciallyimportant substrates makes it suitable for thin, functional coatings.Deposition of thin, transparent, adherent coatings on glass slides bydip coating has been demonstrated.

The pre-ceramic polymers exhibit high yield which allows littleshrinkage related problems. Currently practiced processes for fiberproduction require either fine powder addition and/or slow heating tocontrol shrinkage.

Binders currently employed in powder processing are fugitive organics,such as poly vinyl alcohol, which are pyrolyzed during heat treatment.Because of its high char yield and suitable rheology, the pre-ceramicpolymers of the present invention act as processing aids during formingbut as fillers during sintering to decrease the overall shrinkage ofbulk ceramics.

Because of their molecular nature, the pre-ceramic polymers can beconsolidated and partially sintered to form ultrafine pore filters.

The present invention provides the following advantages overconventionally prepared polymers:

To the best of the inventors' knowledge there are no traditionallyprepared pre-oxide polymers that are water soluble.

Yield is one of the most critical features of a polymer since it has adirect consequence on near-net shaping. The pre-ceramic polymers of thepresent invention have oxide char yields higher than any of thepreviously known polymers. The alumina precursor polymer, for example,has a char yield near 80%. Prior to the present invention, the highestyield ever reported was 40%.

Because the metal salts employed for this process are readily availableat low cost and the only processing equipment required is a ventilatedoven, the cost of the polymer is low.

Unlike metal alkoxides, no polymerization is required for rheologycontrol. Rheology is precisely controlled by simply adjusting the onlyrheology controlling parameter: concentration. Such precise rheologycontrol is impossible with metal alkoxides.

While specific embodiments of the invention have been shown. anddescribed in detail to illustrate the principles of the presentinvention, certain modifications and improvements will occur to thoseskilled in the art upon reading the foregoing description. It is thusunderstood that all such modifications and improvements have beendeleted herein for the sake of conciseness and readability but areproperly in the scope of the following claims.

We claim:
 1. An improved method for making ceramic fibers from a watersoluble pre-ceramic polymer, comprising the steps of:making awater-soluble pre-ceramic polymer, the water soluble pre-ceramic polymerbeing produced by a polycondensation reaction due to heating hydratedmetal salts above their melting points and holding in a well-ventilatedoven to drive off water and ligands; making a water soluble solution bydissolving the water soluble pre-ceramic polymer in heated water;forming fibers from the water soluble solution; drying the formed fibersby heating the fibers in an oven at a rate of about 1° C./minute to afirst temperature ranging between 600° C. to 750° C.; raising the firsttemperature to a second temperature ranging from 800° C. to 1000° C. atabout 5° C./minute; increasing the second temperature to a thirdtemperature of about 1200° C. at about 10° C./minute and holding at thethird temperature of about 1200° C. for about one hour; and cooling toroom temperature at a rate of about 5° C. to 30° C./minute.
 2. Animproved method as recited in claim 1, further comprising the step ofadding a member selected from the group consisting of a nitratestabilized zirconia sol, an acetate stabilized zirconia sol, Y₂ O₃stabilized ZrO₂, and Y₃ Al₅ O₁₂ to the pre-ceramic polymer solutionprior to the forming step.
 3. An improved method as recited in claim 1,further comprising the step of concentrating the water-solublepre-ceramic polymer solution with heat to a viscosity exceeding 10⁶centipoise at room temperature prior to the forming step.
 4. An improvedmethod as recited in claim 1, wherein the first temperature is 600° C.5. An improved method as recited in claim 1, wherein the secondtemperature is 800° C.